I bought my first duplex. give eviction notice or new lease?

[Sinil Kim]

Yesterday I closed my first investment property in Ridgefield, NJ.  

It's duplex.  Each unit has one bed and one bath. 

Average rent for one bed room around the property is $1,200 to $1,400. 

But still the tenants living in the property with low rent. Previous owner didn't raise rent since 2012. 

Current tenants are month-to-month. They don't have any lease since 2013

I want to ask them to sign on new lease with market price like $1200 or leave

They might not sign on new lease with much higher rent. But I want ask them to pay. Or give them eviction notice.

What is nicest way to communicate with them? 

Is there a formal letter? or form or anything.

Someone said if I say something not proper, I can get sued. I don't know what is not proper.

I am just very very new landlord. I don't know nothing about being a landlord.

I need your advise.

Thanks